How to enable the root user in Mac OS X using NetInfo, or change root's password:

1. Open the NetInfo Manager utility (located under /Applications/Utilites/).
2. Click the lock (padlock icon) in the NetInfo Manager window.
3. Enter an administrator account name and password, then click OK (this isn't Administrator as in root, it's Administrator as in a user account with administrative privileges, typically the first account created during the initial install or setup).
4. For Mac OS X 10.2 and later, choose Enable Root User from the Security menu.
5. If you have not previously set a root password, an alert box may appear that says "NetInfo Error," indicating that the password is blank. Click OK.
6. Enter the root password you wish to use and click Set.
7. Enter the password again for verification and click Verify.
8. The root user is now enabled.
9. Click the lock again to prevent changes.